research: small mutation- and property-testing pilot on frontend pure-logic modules

[Xore]

What

Line coverage (#3318) shows code was executed, not that the tests would catch a change to it. OmniRoute runs nightly mutation (Stryker), mutation-redundancy review and property tests (fast-check).

Proposal (pilot, time-boxed)

• Stryker on one or two pure modules (for example src/lib/prefs.ts, the appearance-cookie parsing and serialization). Record killed/survived/timeout.
• fast-check round-trip property for the same cookie parse/serialize pair, with a fixed seed and a small run count in PR CI and a large count on a schedule.
• Decide from the results whether a ratchet is worth it. No gate until then. Only after frontend-next: measure real test coverage and add a non-regression ratchet (no coverage is collected today) #3318 has a baseline.

Sources

• .github/workflows/nightly-mutation.yml
• .github/workflows/mutation-redundancy.yml
• .github/workflows/nightly-property.yml
• docs/ops/QUALITY_GATE_PLAYBOOK.md

Found by the #3194 OmniRoute ops/CI deep-check (pinned 18bbb101, APIARY main 3dca4457).

[Xore]

Resolved by a merged PR (#3412 / #3402 / #3372 / #3389 / #3393). Closing.